Plasticating screw designs for plasticizing plastic resin are many and varied. Designs that include mixing features and features for separating the melt from the unmelts are relevant to the present invention.
U.S. Pat. No. 4,330,214 to Willert teaches a plasticizing screw having a feed zone, a plasticizing zone and a metering zone. A second flight is introduced partway along the screw to provide a means of separating the melt from the unmelts. The melt pool flows over a spill flight and accumulates in a gradually deepening channel between the spill flight and a downstream conveying flight while the unmelts remain in a gradually shallowing channel between the spill flight and an upstream conveying flight. Willert does not include mixing features in the screw.
U.S. Pat. No. 3,870,284 to Kruder and later U.S. Pat. No. 5,219,590 to Kruder and Calland teaches a wave screw with a mixing section. The mixing section consists of a conveying flight and two divider flights each spaced from the conveying flight to divide the channel into three channels of varying cyclic depth in the helical direction of the channels. The portions of minimum depth of the channels define wave crests that are helically displaced from one another. The rotation of the screw causes a kneading-like action on the resin in the mixing section of the screw.
U.S. Pat. No. 6,227,692 to Heathe teaches a plasticating screw that comprises a feed zone, a compression zone, a waved melting zone, a spiral mixing zone that traps large agglomerations and contaminants and a second melting zone.
U.S. Pat. No. 6,132,076 to Jana et al teaches a vented screw with “undercut” spill flights to increase dispersive and distributive mixing. FIG. 2 lists a variety of prior art mixing designs that are typically used in combination with plasticating screws. One example is the “pineapple mixing section” at the screw tip.
U.S. Pat. No. 3,687,423 to Koch et al teaches a plasticating screw with a variable width channel having multiple cross channel dams that impede the flow of unmelts along the channel. The conveying flights have notches cut through them adjacent some of the dams to allow unmelts to move downstream and relieve pressure against the dam wall. The design creates large pressure drops in the barrel and creates dead spots next to the dams where resin can be trapped and degrade.
U.S. Pat. No. 4,107,788 to Anders teaches an extruder screw having a mixing section partway along the screw. The mixing section has multiple start conveying flights forming channels. Each channel has two dams with notches in the conveying flights at the dam sites and immediately upstream therefrom.
U.S. Pat. No. 4,639,143 to Frankland teaches an extrusion screw with a section partway along the screw having three parallel sets of grooved recesses in the melt channel that are designed to reduce the average shear heating effect and consequently, the material temperature, without reducing throughput. The design allows viscous materials to stagnate in the recesses, as there is no means to flush the recesses with fresh material.
U.S. Pat. No. 4,840,492 to Nakamura teaches a mixing screw having a mixing section partway along the screw. The mixing section has a series of recesses of varying width and varying channel depth to provide a combined mixing and kneading action. The design allows viscous materials to stagnate in the recesses, as there is no means to flush the recesses with fresh material.
U.S. Pat. No. 3,941,535 to Street teaches an extrusion screw having a section partway along the screw having notches in the conveying flights. There is no teaching of guiding the melt to pass through the notches and consequently material may become trapped in the notches and consequently degrade.
An overly aggressive plasticizing screw not only creates high levels of shear and degradation but also can generate so much heat in the melt by the shear heating effect of the screw that the barrel temperature will rise and exceed the set points for the barrel heaters and consequently trip overheat alarms causing a shutdown. These problems can arise when attempting to increase the throughput of any given size conventional plasticating screw.
While these problems may be overcome by providing larger or longer screws to increase the throughput of the screw, such solutions are expensive to implement and increase the space requirements of the plasticating unit. It is preferable to find a way to increase the throughput of the plasticating screw without changing its dimensional characteristics. The present invention achieves this objective by providing an improved mixing section that keeps agglomerate from passing through the screw, prevents the build-up of trapped material and ensures melting of all material in the screw without an unacceptable increase in temperature in the melt channel.